The disclosure relates to a transmission combination, a traction drive therewith, and a method for calibrating a clutch of the transmission combination.
In order to increase the transmission ratio range of a traction drive, for example hydrostatic transmissions with two hydraulic motors which are operated fluidically in parallel are known. The drive shaft power thereof can be added via a compounding transmission of the hydrostatic transmission and transmitted, for example, to a vehicle axle. For example, at a low velocity both hydraulic motors are in parallel operation and therefore permit a high traction force. Given a predefined delivery quantity of the hydraulic pump and against the background that the reduction of the swept volumes of the hydraulic motors entails a decreasing efficiency level, the velocity which can be achieved with two motors is limited.
In order to obtain a driving range of a relatively high velocity, one of the hydraulic motors can therefore be set to expulsion volume zero and be disconnected from the output by means of a clutch. The entire volume flow of the hydraulic pump is therefore directed via the remaining, usually smaller hydraulic motor, which permits relatively high rotational speeds and therefore relatively high velocities.
If both hydraulic motors then contribute again to the transmission of power, the clutch must be closed. For this purpose, the hydraulic motor which is previously disconnected from the output has to be accelerated from a stationary state to its working rotational speed. This acceleration process is to take place both quickly and as far as possible without being noticed by the driver. Owing to these, to a certain extent, contradictory requirements, the control of the coupling process is demanding.
The clutch is activated, for example, by means of a hydraulic cylinder. Depending on the design, the closing of the clutch is brought about here either by the application of pressure medium to the hydraulic cylinder or by the discharging of pressure medium from the hydraulic cylinder. Charging and discharging can be referred to as supplying pressure medium.
In principle, the closing process of the clutch can be divided into two phases. In a first phase, the pressure medium is supplied in such a way that the clutch is engaged as quickly as possible, that is to say the clutch stroke is implemented as far as the grinding point in the shortest possible time. The second phase starts from the grinding point, that is to say if torque is being transmitted by the clutch. Starting from this point, the supply of pressure medium mainly only then brings about an increase in the transmissible torque of the clutch and no longer a clutch stroke.
The first phase is optimally very short in terms of time and ends in a position directly before the grinding point. The second phase is then preferably controlled in such a way that the respective hydraulic motor is accelerated in the desired gentle fashion.
The prior art discloses switching valves in combination with nozzles for supplying pressure medium to the hydraulic cylinder of the clutch and therefore for controlling the two phases. Although it is possible to map the torque profile of the clutch gently during its closing, a rapid piston movement of the hydraulic cylinder in the first phase is virtually impossible, in particular in the case of cold oil. Although in this case the supply of pressure medium is enhanced in both phases through the use of relatively large nozzles, which shortens the switching process, this also makes the switching process less comfortable.
As an alternative to switching valves, a proportional valve can be used to resolve this problematic situation. However, the grinding point then has to be known as a function of a process variable which can be saved in the transmission controller. At present, for this purpose, the grinding point is determined manually, and therefore calibrated, on a roller test stand once, before the delivery of the traction drive or the vehicle.
A disadvantage of this solution is, on the one hand, that the calibration and setting on a roller test stand is complex. In addition, owing to the natural change in the coupling characteristic due to wear the re-calibration requires a return to the roller test stand every time. Since this entails a high degree of expenditure, and therefore is frequently dispensed with, the switching comfort of such clutches is reduced markedly over the service life.
In contrast to this, the disclosure is based on the object of providing a transmission combination which can be calibrated with little expenditure, and a traction drive therewith. Furthermore, an object of the disclosure is to provide a method for controlling, in particular for calibrating, the transmission combination.